How to create an iRule with that cascades 3 sequential failover pools

Hi everyone

I am trying to create an irule where, if the default pool fails - I drop into a standby pool, and if that one fails another web pool and finally a last hop pool

I am trying to put it together like this but there is a problem with my syntax which I have not yet gotten to work.

when HTTP_REQUEST { 

 if { [active_members [LB::server pool]] < 1 } {

   pool pool_standby

 } else {

  if { ([active_members webcache_pool_standby] ==

 0) } 

  pool next_backend

 else 

 { if ([active_members web_backend] ==

 0)}

 pool last_pool

 }

}

Any help would be appreciated.

Thanks

Chung